Sometimes an idea is so simple, you assume it must have been done before. Tubenav is one such. It uses the tube map as an intuitive interface for discovering places to go out.
Say you're looking to meet friends in Hammersmith, but none of you really know the area. Click 'Hammersmith' on the tube map and you'll find dozens of suggestions for places to go. You can then filter down by category. 13 pubs are suggested; 11 coffee shops; three cocktail bars; one vegetarian restaurant... and so on... and all are marked with the walking distance from the station. You can then tap through for more info on the venue.
The database is powered by FourSquare, which means you should get plenty of up-to-date recommendations (20,000 and counting). The apps even build in navigational advice, including live tube disruptions. It's available as a website, or as free download for iOS and Android (5-star rating).